An interesting document. Presumably it has been sent to all potential SM scheme designers as well as all HE MSPs in each Area?
A few comments if I may, in no particular order:
IANs 150 and 163 have been withdrawn, so referencing them in this document might cause problems for anyone who doesn’t still have a copy. I’m also not sure what the legal position would be if we do quote withdrawn documents. However, I understand that the contents of these IANs have been subsumed into TSM Chapter 8, Part 3, currently under revision prior to hard copies being printed by TSO. Ultimately I understand that Part 3 will be wrapped into revised Parts 1 and 2, so further editing of this document may be advisable in future?
There are numerous examples of “should” and “should not” statements throughout the document, which is fine, but I’d add a cautionary note that there will inevitably be sites where whatever the advice might be just can’t be accommodated. For example, siting a wicket sign adjacent an entry or exit slip is a very common occurrence with static plate signs and given the potential difficulties of siting FTPs, I would not be at all surprised to find this to be reasonably common for ROTTM signs.
I’m struggling to understand the emphasis on “Regularity” and how it might influence the design. An array of ROTTMS typically comprises 5 nearside only signs. I fear it will be quite common for traffic travelling in anything but LBS1 to find one or more of these signs obscured by HGVs travelling in LBS1. However, when we consider that these LED ROTTMs tend to be much more visible than static plate signs, I think we can be reasonably confident the approaching drivers will be adequately warned of the closed lane(s) ahead no matter how regularly or irregularly they actually observe any unobscured signs. I note that only having signs on one side of the road may well lead to obscuration regardless of whether they are plate signs or LEDs. Depending on traffic flows, we can never be certain that placing signs on only one side of the road might result in some drivers not seeing any signs on the approach to a taper point – although I’d like to think that a taper lit by sequential lamps will be visible even if the approach signs all happen to be obscured by HGVs.
I would expect a designer to attempt to site all the wicket signs at a certain spacing, then adjust them all if it proves that one of them is in a sub-optimal location, should that one sign require moving by a significant degree, whilst still keeping within the 10% siting tolerance for all signs. This should be a rare occurrence and might mean that a totally different FTP could be called for instead. If a design had, say, the 800 and 600 signs very close together but the 400 then a “long” way after the 600, I’d hope the designer would make all attempts to make the spacing more regular without needing to worry about producing a chart to prove they’ve done so. Or simply move the FTP point a few metres if possible, to allow the approach signs to then be “correctly” located? Note that static plate signs tend to be placed 2 marker posts apart, (very approximately 200m), allowing that the distance between marker posts all too often is a movable feast. I’m not aware of anyone in the TTM industry ever siting signs on motorways based on the yardage quoted on a discrete sign, in my experience they are always simply sited based on marker posts.